The present invention relates to a process for producing high knot strength polyamide monofilaments.
Polyamide monofilaments (hereinafter referred to briefly as monofilaments) are preferably used for fishing lines and fishing nets due to their toughness, transparency and flexibility. In these uses the monofilaments are often knotted. Because the monofilaments tend to break at these knots, improvement in knot strength is required.
If the knot strength of the monofilament could be improved enough, the monofilament could be used in a significantly finer denier. This would also significantly improve the transparency and flexibility of the monofilament and products such as fishing line and fishing net could be produced which would have excellent performance.
For this purpose, a special structure has heretofore been proposed (U.S. Pat. No. 3,595,952). Drawn monofilament is heat-treated with saturated or wet steam, and for improving knot strength attention is paid to the condition of the skin layer. However, this process has still room for further improvement for stably obtaining monofilament having a knot strength of not less than 5 g./denier, preferably not less than 6 g./denier, in a denier up to about 3,000. It does not respond to the requirements of those skilled in the art as a process for drastically improving knot strength.
The process of U.S. Pat. No. 3,595,952 modifies to some extent the elastic properties of the skin layer of the monofilament, but does not cause sufficient molecular orientation in the inner layer of monofilament to provide sufficient endurance against load.
We have observed that, when knots were made of monofilament and the knotted monofilament was pulled, portions of the skin layer of the monofilament were subjected to extension and other portions to compression. We have found that a monofilament having high knot strength had very special properties. It must have strengths enabling the skin layer of the monofilament to cope with deformation due to extension and contraction as well as compression. We refer to this property hereinafter as the elasticity of the monofilament. This elasticity should be great.
We have also found that the monofilament should be endowed with properties enabling the inner layer or core of the monofilament to withstand stresses in a direction along the length of the monofilament, such stresses being applied to the monofilament per se. These inner layer properties are hereinafter referred to as endurance or load strength, which should be high.
We have accordingly discovered that a significant increase of monofilament knot strength may be achieved by causing the skin layer of the monofilament to form a specified molecular structure for developing the aforesaid elastic property and by also causing the inner layer of the monofilament to acquire a high molecular orientation for increasing endurance to load, and that this can be achieved by causing the monofilament to form different phases of heterogeneous sectional structures.